The North's agreement to freeze some nuclear and missile activities in return for massive US food aid has raised cautious hopes of eased tensions under its new young leader Kim Jong-Un.
In statements released late Wednesday announcing the deal, both Pyongyang and Washington pledged to work for better relations.
But Friday's comments from the North's supreme military command struck a different tone with the South.
The command accused South Korean troops of displaying slogans or placards slandering the North's top leaders at their barracks, shooting ranges and other military facilities.
The soldiers "openly slandered and defamed the dignity of the supreme leadership of the DPRK (North Korea) after creating a touch-and-go situation", it said in a statement on the official news agency.
The command "solemnly declares once again that it will indiscriminately stage its own-style sacred war to wipe out the group of traitors".
"Those who hurt the dignity of the supreme leadership of the DPRK even a bit will find no breathing spell in this land and sky," it said.
The command vowed to "mercilessly" wipe out anyone who "slightly insults and defames" the dignity of the North's supreme leadership.
Pyongyang made similar threats last year when South Korean reservists were found to be using pictures of the ruling Kim dynasty as rifle-range targets. The South says that practice has been stopped.
Pyongyang has taken a consistently hostile tone towards Seoul since Jong-Un took over after his father Kim Jong-Il died of a heart attack on December 17.
Last Saturday it threatened a "sacred war" over US-South Korean joint military drills, describing them as a "silent declaration of war".
The North vows never to deal with the South's conservative leaders, accusing them of trying to spark a war and rejecting appeals for dialogue. On Friday it compared them to "a mad dog getting more ferocious before meeting its end".
But Seoul's Unification Minister Yu Woo-Ik told a forum Friday: "We urge North Korea again to quickly respond to our proposal for dialogue."
Under the North's agreement with the United States, the communist state promised to suspend a uranium enrichment programme and declare a moratorium on nuclear and long-range missile tests. It would also re-admit UN nuclear inspectors.
The United States said it would provide the impoverished and hungry country with 240,000 tonnes of food intended for young children and pregnant women.
On Friday the US State Department said US and North Korean officials will meet next week in Beijing to finalize plans for the food aid.
Victoria Nuland, a State Department spokeswoman, said Robert King, the US envoy on human rights in North Korea who has been the pointman on food assistance, will meet in the Chinese capital on Wednesday for talks with a counterpart from Pyongyang.
"The idea is to finalize all of the technical arrangements so that the nutritional assistance can begin to move," Nuland told reporters.
"My understanding is we're down to issues like what port, when, who manages it, how do we count, how do we monitor," she said.
During her regular briefing Nuland added that the North's statement threatening "sacred war" was "unfortunate".
"Frankly, it's not helpful to the kind of environment that we're trying to foster," she said.
The nuclear deal follows US-North Korean talks last week in Beijing aimed at restarting six-party negotiations on the North's nuclear disarmament.
China, the North's sole major ally and economic prop, hosts those talks which also group the two Koreas, the United States, Japan and Russia.
Chinese Foreign Minister Yang Jiechi met his South Korean counterpart Kim Sung-Hwan on Friday before visiting President Lee Myung-Bak.
The ministers discussed the nuclear deal and other issues, Kim's office said, without elaborating further.
Yonhap news agency quoted an unnamed South Korean official as saying that Kim and Yang welcomed the deal and agreed to work together to encourage Pyongyang to take "concrete actions" to implement its pledge.
China has welcomed the deal and pledged to push ahead with efforts to revive the wider nuclear dialogue.
The disarmament talks have been stalled for some three years. The disclosure in November 2010 of the North's enrichment programme, which could give it a second path to an atomic bomb, has lent urgency to the diplomacy.
The United States and its allies had demanded a shutdown of the programme as one precondition for reviving the six-party forum.
Washington had also called on Pyongyang to improve ties with Seoul, although the State Department made no mention of this in its statement Wednesday.
Amid the latest progress, the United States said North Korea's top nuclear negotiator Ri Yong-Ho would pay a rare visit next week for talks at a US university.
Analysts said Wednesday's deal could help revive the six-party talks, but many remain sceptical that the North will abandon its nuclear weaponry.
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